An ink jet printhead is one example of a fluid ejection device. Applications include, but are not limited to printers, graphic plotters, copiers and facsimile machines. Such apparatus use an ink jet printhead to shoot ink or another material onto a medium, such as paper, to form a desired image. More generally a fluid ejection device is a precision dispensing device that precisely dispenses fluids such as ink, wax, polymers or other fluids. While printing to form an image on a surface is a well known application, fluid ejection devices are not limited to this and may be used for other purposes, such as manufacturing or 3D printing for instance.
Fluid ejection devices may eject the fluid by any suitable method, for instance thermal expansion of the fluid or a piezo-electric pressure wave. A thermal fluid ejection device typically heats a resistor causing fluid in a chamber near the resistor to evaporate and form a bubble. Pressure from the bubble causes fluid to be ejected through a nozzle of the fluid ejection device.
It can be useful for a fluid ejection device to be able to generate different sizes of fluid droplet. Smaller fluid droplets can be used for high resolution, while larger fluid droplets may be used to efficiently cover larger areas for instance. The size of the fluid droplet ejected through the nozzle depends, inter-alia, on the size of the resistor. A larger resistor will in general generate a larger bubble displacing more fluid and thus produce a larger fluid droplet. Some fluid ejection devices have two different sizes of resistor in order to produce two different sizes of fluid droplet. However, having two different sizes of resistor takes up a lot of space while only limited space may be available on the fluid ejection device.